As integration densities of integrated circuit devices continue to increase, it may become increasingly difficult to fabricate fine linewidths using conventional photomasks. Photomasks are conventionally used to expose photoresists according to a predetermined pattern. The photoresist is used to pattern an underlying layer such as a semiconductor substrate, or a conductive or insulating layer. Conventional photomasks may be limited in defining fine linewidths for highly integrated devices. Accordingly, phase shift masks are being used as an alternative for increasing integration density.
In contrast with a conventional transparent photomask, the phase shift mask operates on the principle that radiation such as light having different phases can interfere. For example, if radiation such as light from a light source passes through adjacent slits, the light emerging from the slits has different phases that can mutually interfere. When the value of the phase difference satisfies a predetermined criteria, mutual destructive interference can occur between the light. A photomask using this interference principle is generally referred to as a "phase shift mask".
Phase shift masks can therefore offer increased resolution and improved depth of focus compared to conventional photomasks. Thus, very fine patterns can be formed compared to conventional photomasks. Phase shift masks are especially useful in forming repeated line-space patterns where phase shifts of 180.degree. between adjacent apertures can produce cancellation of light.
A major type of phase shift mask currently being used is referred to as a "Levenson" mask. The Levenson phase shift mask is described in a publication entitled "Optical/Laser Microlithography VIII", SPIE Proceedings, Volume 2440, Feb. 22-24, 1995, pp. 34-36, and U.S. Pat. No. 5,358,829, issued Oct. 25, 1994 to Garafalo et al., entitled "Phase-Shifting Lithographic Masks With Improved Resolution", the disclosures of which are hereby incorporated herein by reference. There are generally two fabrication methods for Levenson phase shift masks. The first involves etching of a substrate and the second forms a phase shifting layer on a substrate.
In the first method, a photomask substrate such as a quartz substrate is etched in a predetermined pattern. Thus, phase differences are generated based on the principle that incident light which passes through the etched and unetched portions of the photomask substrate will have different path lengths and therefore will have different phases.
In phase shift masks which are fabricated by forming a phase shifting layer on a substrate, a spin-on-glass (SOG) layer is generally coated on a photomask substrate and then patterned, thereby exposing predetermined portions of the substrate. This type of phase shift mask is also referred to as an SOG-coated phase shift mask. Phase differences are created by radiation passing through the portion of the substrate which includes the patterned SOG coating relative to the exposed portions of the substrate.
Although phase shift masks which include phase shifting layer patterns can form finer linewidths than conventional masks, there continues to be a need for forming ever-finer linewidths due to the ever-increasing integration density of microelectronic devices.
Referring now to FIGS. 1A-1C, a method for fabricating a conventional SOG-coated phase shift mask will now be described so as to illustrate problems which are encountered with conventional phase shift masks. In particular, as shown in FIG. 1A, a radiation blocking layer pattern 15 is formed on a phase shift mask substrate 10. The radiation blocking layer pattern 15 defines a predetermined portion h of the phase shift mask substrate. The radiation blocking layer pattern may be formed by blanket depositing a radiation blocking layer such as chrome (Cr) on the substrate 10. A photoresist pattern is then formed to expose a predetermined portion of the radiation blocking layer by coating a photoresist on the radiation blocking layer and patterning the coated photoresist. The radiation blocking layer is then patterned by removing a predetermined portion thereof using the photoresist pattern as an etch mask. Then, the photoresist remaining on the radiation blocking layer 15 is removed.
Referring now to FIG. 1B, a phase shifting layer 20 is formed on the substrate 10 including on the radiation blocking film pattern 15 and on the substrate between the radiation blocking film pattern 15.
As shown in FIG. 1C, a phase shifting layer pattern 20a is formed from the phase shifting layer 20, to thereby define the substrate into phase shifting layer-coated areas and phase shift layer-free exposed areas. Phase shifting layer 20 may be patterned by coating a photoresist on the phase shifting layer 20, patterning the photoresist, patterning the phase shifting layer 20 using the patterned photoresist as a mask and then removing the remaining photoresist from phase shifting layer pattern 20a.
Although phase shift masks as described in connection with FIGS. 1A-1C can provide improved photomasks relative to conventional photomasks, it may be desirable to form even finer patterns for integrated circuits than can be formed with the above-described phase shift masks.